The present application is based on Japanese priority application No.2000-216387 filed on Jul. 17, 2000, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to semiconductor devices and more particularly to a high-power and high-speed semiconductor device.
With widespread use of mobile telecommunication technology, there is a demand for high-power and high-speed semiconductor devices for use in base stations as final stage amplifiers, and the like.
Conventionally, high-power semiconductor devices have been realized by increasing the gate width so as to increase the drive current. However, such an approach has a drawback, associated with the increased output current, of large power loss occurring in an impedance matching circuit that is used in combination with the semiconductor device. In view of this problem, recent high-power semiconductor devices achieve the desired increase of output power by increasing the operating voltage.
FIG. 1 shows the construction of a conventional high-power, high-speed semiconductor device 10.
Referring to FIG. 1, the semiconductor device 10 is a MESFET formed on a semi-insulating GaAs substrate 11, and includes a buffer layer 11A of undoped GaAs formed on the GaAs substrate 11, a channel layer 12 of n-type GaAs formed on the buffer layer 11A, a Schottky contact layer 13 of undoped AlGaAs formed on the channel layer 12, and a cap layer 14 of undoped GaAs formed on the Schottky contact layer 13. Further, a gate electrode 15 makes a Schottky contact with the Schottky contact layer 13 in a gate recess structure formed in the cap layer 14, and n+-type diffusion regions 16 and 17 are formed at respective sides of the gate electrode 15 with a separation therefrom. Each of the diffusion regions 16 and 17 extends from the cap layer 14 to the buffer layer 11A and forms a source region or a drain region. Further, a source electrode 16A is formed on the source region 16 in ohmic contact therewith, and a drain region 17A is formed on the drain region 17 also in ohmic contact therewith.
In the MESFET 10 of FIG. 1, the exposed part of the cap layer 14 is covered by a passivation film 18 of SiN.
In the case the MESFET 10 is driven so as to provide large output power, it is necessary to apply a large voltage between the gate electrode 15 and the drain electrode 17A.
On the other hand, the use of such a large operational voltage tends to cause the problem of excessive electric field strength in the channel region formed underneath the gate electrode 15, particularly in the vicinity of the drain edge. The large electric field thus induced in the vicinity of the drain edge may cause the problem of avalanche breakdown in the channel region as represented in FIG. 2. When this occurs, a large gate leak current is caused to flow along a path (1) as represented in FIG. 2, and the desired high-power operation of the MESFET 10 becomes no longer possible.
Further, there may exist another leakage current path (2) in the conventional MESFET 10 of FIG. 1 as represented in FIG. 2, although the magnitude of the leakage current along the path (1) is larger than the leakage current along the path (2) by the factor of ten or more.
In order to avoid the problem of gate leakage current, it has been practiced conventionally to increase the distance between the gate electrode 15 and the drain electrode 17A so as to reduce the electric field strength right underneath the gate electrode in the pinch-off mode. According to this approach, it is confirmed that there occurs a desired increase of the gate-drain breakdown voltage and also a desired decrease of the gate leakage current.
On the other hand, the foregoing conventional approach still has a drawback in that, while it can successfully increase the gate-drain breakdown voltage, there also occurs an increase of the source-drain resistance, resulting in a decrease of the maximum output current, and hence a decrease of maximum output power that can be taken out from the semiconductor device. Further, the approach of increasing the gate-drain distance tends to cause the problem of Gunn oscillation.
From the reasons noted before, it will be understood that there exists an inherent limitation in the foregoing conventional approach for realizing high-power operation of MESFET 10 of FIG. 1.
Accordingly, it is a general object of the present invention to provide a novel and useful semiconductor device wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a high-speed compound semiconductor device capable of providing large output power.
Another object of the present invention is to provide a high-speed compound semiconductor device operable at large output power with minimized leakage current.
Another object of the present invention is to provide a compound semiconductor device, comprising:
a substrate;
a channel layer formed on said substrate;
a cap layer formed on said channel layer;
an insulating film formed on said cap layer;
a gate recess opening penetrating through said insulating film and said cap layer;
an n-type source region extending from a surface of said cap layer and reaching said channel layer at a first side of said gate electrode;
an n-type drain region extending from a surface of said cap layer and reaching said channel layer at a second side of said gate electrode;
a source electrode contacting with said source region electrically; and
a drain electrode contacting with said drain region electrically,
said gate electrode having a xcex93 shape and extending over said insulating film from said gate recess opening in a direction of said second side,
a total thickness of said insulating film and said cap layer being set such that there is formed an electric field right underneath an extending part of said gate electrode such that said electric field has a component acting in a direction perpendicular to a principal surface of said substrate with a substantial magnitude.
According to the present invention, it becomes possible to improve the gate breakdown characteristics of a high-speed field-effect semiconductor device by providing thereto a xcex93-shaped gate electrode and by optimizing the thicknesses of the passivation film and the cap layer such that the shape of the gate electrode can deform the potential distribution profile in the vicinity of the drain edge. As a result of the present invention, it becomes possible to use a large gate-drain voltage and the semiconductor device can be driven so as to provide a large output power.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.